1. Field of the Invention
The present invention relates to an automatic and continuously operating standby A-C power system and more particularly to circuitry included in such a system for compensating for, and thereby minimizing, the effect of the transient which occurs when the system switches from the conventional source of A-C power to the inverter and battery contained therein.
2. Description of the Prior Art
For many applications such as communications and data processing equipment it is desirable to provide a source of standby A-C power. The A-C voltage from a conventional source of power such as a power station is generally used either alone or in combination with a secondary reserve A-C power source such as a battery and an inverter to supply the equipment when that voltage is acceptable. The conventional voltage is, however, subject to power level fluctuations which may take the form of a blackout (complete loss of A-C power), a transient (momentary voltage excursion), an interrupt (momentary loss of power) or a brownout (long-lasting reduction in A-C power). Upon the occurrence of one or more of the conditions described above, the source automatically disconnects the conventional A-C voltage from the load. Power is then supplied to the load only from the inverter and battery contained therein. Upon restoration of the conventional or input A-C voltage the source will reconnect the line to the load and may also provide current for recharging the battery.
In one type of standby source known as the transfer type it is only the conventional A-C voltage which is normally used to provide power to the load external to the source. When the conventional A-C power is operating normally, the reserve source consisting of the battery and inverter mentioned above does not supply power to the load but operates in a standby condition. Upon the occurrence of one or more of the previously described conditions, the source automatically disconnects the conventional A-C voltage from the load and the standby inverter and battery supplies the load power.
In one such type of standby prior art source which is described in U.S. Pat. No. 3,348,060 (hereinafter the '060 patent) the inverter also operates bidirectionally and thereby serves to charge the battery from the conventional A-C when the source operates in its normal mode. In the kind of transfer type source described in the '060 patent it is necessary to control the phase angle between the output and conventional or input A-C voltages as a function of both the external load and the charge on the battery. In this manner current may be supplied from the line voltage for providing power to the external load and for charging of the battery if needed. The battery may be considered to be an internal load on the system, and the combination of external and internal loads will be referred to hereinafter as the total load on the system. Upon restoration of the A-C line voltage after the occurrence of one of the conditions described above, it is also desirable to slowly bring the source's output voltage into phase relationship with the A-C line before the line is reconnected to the load. This slow bringing into phase of the input and output voltages prevents undesirably large amplitude spikes from appearing at the source's output. A continuously adjustable phase-control circuit for use in the source described in the '060 patent is shown and described in U.S. Pat. No. 3,337,743 (hereinafter the '743 patent). A line control for use in the source described in the '060 patent is shown and described in U.S. Pat. No. 3,389,268 (hereinafter the '268 patent).
Since uninterruptible power sources or supplies are frequently used to supply power to data processing type circuits, the reserve power source must act quickly to supply power whenever the primary power source undergoes one of the previously described conditions in order to maintain the integrity of information in the data processing circuit.
The sudden transfer of total system load from the conventional A-C voltage to the A-C voltage generated by the combination of inverter and battery causes a momentary decrease in the amplitude of the source's output voltage. This decrease in amplitude, while relatively short in duration, does give rise to a transient which may cause undesirable effects in the external load. If the external load consists of data processing equipment, the transient may impair the integrity of the data contained therein. Such impairment may not be immediately evident. It is, therefore, desirable to compensate for and thereby minimize any detrimental effect that the transient may have on the external load when the source switches from the conventional A-C power to the reserve supply.